Mechanosynthesis of Nanophase Powders M F

Mechanosynthesis of Nanophase Powders M F. Miani F. Maurigh DIEGM University of Udine, Udine, Italy

INTRODUCTION THE MECHANOSYNTHESIS PROCESS

Among the different processes able to produce nanocrys- The concept of the mechanosynthesis process is very talline powders in bulk quantities, mechanosynthesis, i.e., simple: to seal in a vial some—usually powdered—ma- the synthesis of nanograined powders by means of terials along with grinding media, which are usually balls, mechanical activation, is one of the most interesting from and to reduce the size of the materials by mechanical an industrial point of view. Mechanosynthesis of nano- action, providing at the same time an intimate mixing phase powders is one of the less sophisticated technolo- of the different materials introduced, so close that gies—and, in such a sense, also the most inexpensive—to diffusion and chemical reactivity are greatly enhanced. produce nanophase powders; in fact, it exploits devices For clarity, one could consider a mixture of elemental and processes that have many aspects in common with powders—for simplicity, we could limit to powders of mixing, fine grinding, and comminution of materials. element A and element B—introduced in a cylindrical However, it does have interest for applications, as, in container, the jar of a vibrating mill, with repeated and principle, this is a very low cost process and its potential prolonged impacts imposed by mechanical energy im- for industrial applications is very significant. This article parted to grinding media (Fig. 1). describes the basis of the process, some possible applica- Depending on the nature of the reactants, different tions and perspectives of mechanosynthesized nanophase intermixing mechanisms are possible; generally speaking, powders, and some simple ideas that are able to depict part the ductile phase tends to incorporate—if such a phase is of the process, with a specific mind to the mechanosynth- present—a more brittle phase, with a tendency to form esis of nanocrystalline materials starting from elemen- particles which are microscopically (to a scale of microm- tal powder precursors. Mechanosynthesis belongs to the eters) folded. family of process technologies known as mechanical al- With prolonged milling, the crystal size of both re- loying, which are basically using the same or very similar actants decreases in an exponential way, following a de- apparatuses, i.e., milling or size reduction devices, which creasing trend with the processing time t of the type:[8,9] could be generally described as ball mills. A recent [1] ðÀc1tÞ thorough analysis by Suranarayana of the processes d ¼ df þðdi À df Þe ð1Þ related to mechanical alloying defines mechanosynthesis as a mechanochemical synthesis, attributing most of where d should be considered representative of the crystal the research work in this field of mechanical alloying size, being di the initial, df the steady-state crystal size, to Dodd and McCormick,[2] Takacs,[3] and Matteazzi and c1 an empirical constant depending on the process et al;[4] it is also necessary to quote the fundamental parameter. Other possible trends of the crystal size and pioneering work of Butyagin[5] and Urakaev and evolution with milling time are discussed in the next Boldyrev[6] and some important developments by Calka paragraph, especially if exothermic effects of the reaction and Wexler.[7] While, because of many different process A + B ! AxBy are to be taken into account. parameters and conditions, a complete modeling of the The gradual transformation of the milled powders from mechanical alloying process and mechanosynthesis of a rather coarse to a nanocrystalline grain size could be nanophase materials would be an aim very difficult to simply depicted as a formal reaction: reach, it is useful to identify the operating principle of A þ B ! A nano þ B nano the most common devices employed for the process, along with some kinematic aspects, some specific me- Miani and Fecht[10] have been considering mechanical chanical considerations, and thermodynamic and kinetic milling—i.e., the reduction of metal powders of elements issues of the process as well. Finally, current production of and intermetallic compounds to nanocrystalline powders mechanosynthesized powders and perspectives will be by grinding—and they have proposed empirical estimates briefly discussed. for the enthalpy stored. These estimates could be a base

Dekker Encyclopedia of Nanoscience and Nanotechnology 1787 DOI: 10.1081/E-ENN 120009258 Copyright D 2004 by Marcel Dekker, Inc. All rights reserved. ORDER REPRINTS 1788 Mechanosynthesis of Nanophase Powders

Fig. 1 Mechanosynthesis schematic process and steps. for an evaluation of the mechanochemical energy transfer efficiencies in the process that are decreasing from imparted to the powders. In mechanochemical processing, vibrating to tumbling—have been employed successfully. as time is increasing, along with the energy imparted to The different devices employed for research purposes are the materials, new phases/compounds tend to appear. It described in Koch;[11] there has been a tendency of has been generally recognized, by different authors, that different research groups[5,12,13] to develop ad hoc mills the generic reaction path linking the time evolution of the for the synthesis by milling of nanophase materials. Fig. 2 reactants with time is sigmoidal.[8–10] We shall come back describes a mill developed in the University of Udine, to this point dealing with the process kinetics. Here we which has been scaled-up to be industrially employed. could depict the second step of the reaction as: The most important physical parameters involved are well described in a series of papers by Cocco and nA nano þ B nano ! A B nano n Delogu,[8,9,14] in an attempt to propose a quantitative This is an oversimplification, as the mechanosynthesis understanding of the mechanical alloying processes. process occurs continuously. However, it may be useful to These authors have underlined the importance of relating evaluate, for instance, energetic aspects. basic and phenomenological aspects, suggesting that the framework of solid-state chemistry and chemical kinetics should be the most fruitful to interpret results and devel- KINEMATIC AND MECHANICAL ASPECTS op new trends. For the sake of simplicity, considering a mill with just one ball, and adopting the terminology of Milling devices are of different nature: vibrating mills, Butyagin,[5] one of the pioneers in the field of mechano- planetary mills, attritor mills, and tumbling mills—with chemistry, Table 1 was proposed. With such a situation, it ORDER REPRINTS Mechanosynthesis of Nanophase Powders 1789 could be interesting to adopt the quantitative techniques Table 1 Butyagin terminology for an elementary used in engineering the deformation processes, as it has mechanochemical process: ball milling with one ball been proposed earlier by Courtney and Maurice.[15] In any Physical quantity Expression Units M case, with the aid of simple kinetic trends, the milling 2 time should be related to the energy dose, not only for Impact energy E = 1/2mbvimp J device scale-up, but also for process control. This could Impact frequency N hits/sec 2 help, for instance, in controlling exothermic mechano- Milling intensity I = NE = 1/2Nmbvimp W 2 chemical reactions where combustion appears, imparting Energy dose D = It =NEt = 1/2Nmbvimpt J the completion of a mechanochemical synthesis of Specific dose Dm =NEt/mp J/kg nanocrystalline materials. mb and mp are the mass of the ball and of the powder, respectively. A possible new design for a jar for laboratory milling Source: Ref. [14]. is the one presented in Fig. 3, together with its constructional details. The purpose of the experimental jar is to use a diatermic oil to monitor heat release during pro- arguments should not be of help. However, it is possible cessing. The jar has been produced with integral internal [16] to conceive the most simple reaction in the mechano- channels by means of selective laser sintering and it is synthesis of nanocrystalline materials—the one involving available for experimentation on SPEX 8000 mills; typical elemental powders A and B—as a two-step process: the dimensions of the main cylinder are 75mm in height and first could be thought of as a reduction of the crystal 60mm in outside diameter. domains and the second as the formation of a nanophase compound or alloy. Miani and Fecht’s[10] extrapolation may be employed for the first step, and Miedema model for the second step, assuming that the enthalpy of alloy THERMODYNAMICS formation is the same also in the nanocrystalline status (Table 2). This approach is able to give order-of- Mechanosynthesis is a technique involving far from magnitude correct results, as could be checked compar- equilibrium processing; so, in principle, thermodynamics ing these semiempirical calculations[10] to the actual heat release.[17] Fecht proposed an empirical regression with an analysis by means of differential scanning calorimetry (DSC) data on nanocrystalline metals and intermetallic compounds obtained by 24-hr ball milling by SPEX milling. Scanning up to 870 K with 20 K/min, he obtained the results listed in Table 3. These data allow to estimate the excess enthalpy stored DH:

À5 DH=DHf ¼ 8:5 10 Tm ð2Þ

where DHf is the enthalpy of melting and Tm is the melting temperature.

KINETICS

For what concerns kinetic evolution, the situation is very complex. We have different and contrasting phenomena. Dealing with the mechanosynthesis of nanophase materials, we are interested to obtain a crystal size of the order of tens of nanometers, which is usually considered an effective value for applications. The crystal size evolution Fig. 2 Experimental vibrating mill for the synthesis of nano- with time could be depicted with Eq. 1 reported above. phase materials in significant quantities. (From Ref. [12].) If thermal effects are present, i.e., if the enthalpy of re- ORDER REPRINTS 1790 Mechanosynthesis of Nanophase Powders

Fig. 3 (A) Prototype vial for the SPEX 8000 Mill, along with a section view of internal channels. (B) Prototype vial for the SPEX 8000 Mill: constructional details for the direct metal-selective laser sintering processing.

action A + B is high enough to increase temperature, we Table 2 Miani and Fecht’s[10] data for the empirical should introduce a term: extrapolation of the excess enthalpy stored in mechanically milled nanocrystalline powders @ À E dðt; TÞ¼ f ðtÞþgðtÞeðÞRT ð3Þ @t Material Structure Tm (K) d (nm) DDH (kJ/mol) Fe bcc 1809 8 2.1 where E is an activation energy, R is the gas constant, and Cr bcc 2148 9 4.2 T is the absolute temperature. This leads to an integrated Nb bcc 2741 9 2.1 form of the type: W bcc 3683 9 4.6 Z Co hcp 1768 14 1.0 t À E Zr hcp 2125 13 3.4 d t f u g u eðÞRT du d ð4Þ ð Þ¼ ð Þþ ð Þ þ 0 Hf hcp 2495 13 2.2 0 Ru hcp 2773 13 7.2 NiTi CsCl 1583 5 5 where d0 is the initial grain size. Although this is a hypothesis which neglects chemistry and assumes the CuEr CsCl 1753 12 6.8 SiRu CsCl 2073 7 10.1 existence of just one chemical species or compound, still, AlRu CsCl 2300 8 5.2 this equation is too general to be exploited on a practical ORDER REPRINTS Mechanosynthesis of Nanophase Powders 1791

Table 3 Iron conversion ratio a in the which is still a complicated expression. More insight mechanosynthesis of nanophase iron could be given if the elemental powders are introduced in carbides using a SPEX 8000 mill the vial with an initial ratio m of A on B species equal to M Milling Fe conversion the stoichiometric ratio of the formed compound AnB, n. time (hr) ratio aaa In such a case, e = n/m = 1, and the equation simplifies to:

0.25 12.2 @ 2 aðtÞ¼ð1 À aðtÞÞ f ðtÞ ð7Þ 0.5 12.7 @t 0.75 14.8 1 13.6 which may be integrated as: Z 1.25 14.1 t 1.5 13.2 f ðuÞdu 1.75 13.9 aðtÞ¼Z 0 ð8Þ 2 16.6 t 2.5 26.2 f ðuÞdu þ 1 0 3 26.9 3.5 32.4 Now it is possible to make the position 5 51.8 Z 7.5 68.4 t 10 82.1 F ¼ f ðuÞdu ð9Þ 15 91.0 0 and formally calculate a as a function of F. F a ¼ ð10AÞ ground; however, it does take into account the mechanism F þ 1 of grain reduction as a result of milling action in the term f(t) and the grain growth in the second term. If the reaction It is possible to invert such a relationship to estimate A+B is exothermic enough, adiabatic temperature T, for the unknown function f(t) on the basis of experimen- the sake of simplicity, may be evaluated considering a tal evolution. simple first-principle calculation. In the case the temper- a ature is low enough, the integral equation will coincide F ¼ ð10BÞ 1 À a with Eq. 1. Avvakumov[18] has proposed a kinetic equa- [8,9,14] tion for mechanochemical synthesis; Cocco et al. Coming back to the general solution, one could consider have validated it in some applications of mechanical al- Eq. 6 as a function of F as: loying. One should consider the elements A and B that ðFðe À 1ÞÞ react to form the compound AnB at the initial ratio of A on e À 1 ae ¼ ð11Þ B reactant in the reaction: eeðFðe À 1ÞÞ À 1 nA þ B ! AnB In general, the two F functions will be different; however, it is likely that the f (t) function does not By considering a as the degree of reaction atoms react- depend strongly on composition, as it physically takes ed on the initial atoms, n as the stoichiometric number, into account the mechanical response at a microstructural m as the initial atomic ratio of A atoms on B atoms, and level of the mixture to the repeated impacts of the balls. e as n/m ratio, a generalized Avvakumov equation is This is equivalent to assuming that the crystal size evo- proposed here: lution does not change too much with slight variations of composition. It is then possible, by algebraic substitution, @ to link the more simple kinetics, described by Eq. 8 at aðtÞ¼ð1 À aðtÞÞð1 À eaðtÞÞ f ðtÞ ð5Þ @t initial stoichiometric ratio, to the general described by Eq. 6 in such a way as: With the initial condition a(t = 0) = 0, it is possible to integrate then the equation to: aðe À 1Þ eðÞ1 À a À 1 R R ae ¼ ð12Þ t t aðe À 1Þ À f ðuÞdu þ f ðuÞdue ðÞ1 À a e 0 0 À 1 ee À 1 aðtÞ¼ R R ð6Þ t t À f ðuÞdu þ f ðuÞdue where a may be computed using the values of the mix- À1 þ ee 0 0 ture at initial stoichiometric composition. ORDER REPRINTS 1792 Mechanosynthesis of Nanophase Powders

AN EXAMPLE IN THE Table 4 Iron and carbon crystal size evolution in the MECHANOSYNTHESIS OF Fe3C mechanosynthesis of nanophase iron carbides with a laboratory SPEX 8000 mill

Kinetic quantitative studies are not very common in Fe crystal C crystal mechanosynthesis of nanophase materials. A set of Milling time (hr) size (nm) size (nm) experiments was performed several years ago, and results were published in Ref. [19]. In such a system,[20,21] 0.25 38 24.6 experimental results suggest that fracturing of graphite 0.5 32.5 20.4 crystal progresses by steps: fracturing along the hexago- 0.75 34.2 15.7 nal plane and subsequent fracturing of the hexagonal 1 31.3 10.8 1.25 27.9 networks. Most of the experiments were performed at a 1.5 23.6 fixed initial stoichiometric composition, 3 Fe 1 C; this 1.75 18.1 composition corresponds to that of cementite Fe3C. In 2.5 11.6 such conditions, in the modified Avvakumov Eq. 5 3 13.4 reported above, e = 1 holds, and it is possible to consider 3.5 8.8 the simpler Eq. 7 and its solution Eq. 8. Using a SPEX 5 9.3 8000 vibrating laboratory mill, with 6 g of powder charge and a ball-to-powder weight ratio 10:1, it was possible to identify by appropriate microstructural analysis— to reaction rates in mechanosynthesis would be found in Mo¨ssbauer spectroscopy and X-ray diffraction—several the future, linking f(t) to the crystal size evolution. phases, Fe, C, a low carbon martensite-like alloy, and Considering such a hypothesis, Eq. 7 could become: three main types of carbides. If we simplify the kinetic trend considering Fe3C, cementite, as the unique carbide @ cð1 À aðtÞÞ2 present, and we neglect the martensite-like alloy, we aðtÞ¼ ð13Þ @t d have to take into account just three chemical species: Fe, C, and Fe3C. In such a way, the real kinetic evolution which one may solve as: is simplified; experimental results are reported for iron Z in Table 3. t 1 du The F(t) function presented above is shown as 0 dðuÞ aðtÞ¼Z ð14Þ in Fig. 4. t 1 1 The R2 value for a quadratic fit is high indeed and a du þ 0 dðuÞ c good correlation is found linking [a/(1Àa)] to F(t). The almost parabolic trend of F(t) means that the derivative The crystal size evolution d(t) may be evaluated by means of the F(t) function, f(t), could be linear. It is possible of X-ray diffraction line broadening; experimental results that an approach decoupling the two main contributions for these specific systems are presented in Table 4. One might check that the crystal size evolution could be assumed as a simple reciprocal function of time. In this case, 1/dFe = 0.0259t + 0.0111 and 1/dC = 0.0682t + 0.0189, where dFe and dC are the iron and carbon crystal size, respectively. The reciprocal of the grain size d could be assumed as[10] 1/d =1/dFe + 1/dC and, in this case, by summing 1/dFe and 1/dC contribution, as 1/d = 0.0941t + 0.03. Substituting this expression in Eq. 13 and integrating as in Eq. 14, one could obtain:

ctð941t þ 600Þ aðtÞ¼ ð15Þ 941ct2 þ 600ct þ 20 000

where c should be evaluated by data fitting. With the Fig. 4 Mechanosynthesis of nanocrystalline iron carbides: plot choice c = 0.81, the data fitting is excellent, apart the first of Eq. 10B suggests a possible parabolic trend of F(t) function; three or four points of the kinetics; this could be a result squares are experimental data. (View this art in color at of an erroneous attribution of the hyperfine field distri- www.dekker.com.) bution in the Mo¨ssbauer spectra at early milling times. ORDER REPRINTS Mechanosynthesis of Nanophase Powders 1793

development of a vibrating mill[12,13] scaling up the possibilities of mechanosynthesizing of the SPEX 8000 mill. Latest developments include the study of grain M growth and of consolidation by means of cold isostatic pressing (CIP) and hot isostatic pressing (HIP).[31] The Italian company M.B.N.[23] has now developed and scaled-up, with an industrial production capacity of 200 t/years of materials, several of the earlier works by Matteazzi. The company is now focusing on the Mechan- omade1, which is basically a mechanosynthesis technique, and on consolidation technologies such as forging and extrusion with the processes of Mechanoforge1 and Mechano XT1. These techniques have led, for instance, to the production of an Al2O3 doped copper alloy (99% Cu, 1% Al2O3), which has outstanding mechanical properties (yield strength of 800 MPa, ultimate tensile strength of 900 MPa). Fig. 5 Experimental results (in squares) in mechanosynthesis McCormick has obtained the most interesting results of iron carbide compared with values calculated according to in the field of mechanochemical synthesis of nanocrystal- real crystal size evolution (Eq. 15). line materials. With different coauthors and collaborators, along the years, he has started studying the mechanical alloying process, considering further some constructional The agreement with the F(t) function is also excellent., detail of vibrating mills. He has recently exploited some as reported in Fig. 5. results in the field of displacement mechanochemical reactions, which are interesting for the industrial field to patent a specific process—now property of an industrial spin-off of the University of Western Australia, INDUSTRIAL TRENDS AND RESULTS Advanced Powder Technology Pty Ltd.[23]—that has been called mechanochemical processing MCP1. Mech- At the moment of writing this entry, there are two com- anochemical processing consists in solid-state displace- panies that are exploiting successfully the mechanochem- ment mechanochemical reactions caused by collisions ical synthesis of nanocrystalline powders: Advanced between particles and balls inside mills. At the end of the Powder Technology Pty Ltd.[22] of Perth, Western reactions, nanocrystalline powders (grain size10 nm) Australia, and M.B.N., Italy.[23] Both companies have within a soluble salt matrix are obtained. Thermal treat- strong connections with the university field and could be ments, at low or intermediate temperatures, are typically considered as spin-offs of the research group of Matteazzi performed depending on the specific nanocrystalline et al.[4] and Dodd and McCormick.[2] Matteazzi has powders to be obtained. The final step is washing the pioneered the works in the mechanochemical synthesis of salt matrix with appropriate solvents, allowing to obtain nanophase powders, and, along with Le Caer, he was the separated nanoparticles. first to employ the terminology ‘‘mechanosynthesis’’ in The use of a diluent phase may be necessary to: obtaining nanocrystalline carbides and silicides[24] and aluminides.[25] These compounds, and especially car- 1. Avoid combustion reactions during milling. bides, have remarkable industrial applications in the field 2. Reduce the volume fraction of nanoparticles (in this of cutting tool materials. Matteazzi chose the Fe–C way, it is possible to avoid nanoparticles being ag- system as a model for the mechanosynthesis of nano- glomerated). crystalline carbides for the availability of a powerful tool 3. Control the particle size distribution. in microstructural analysis, Mo¨ssbauer spectroscopy,[26] which is applicable to Fe-containing systems. Subse- Comparing it to the more simple mechanochemical quently, he studied alumina-based nanocomposites[27] reaction which has been presented in previous paragraphs, and the solid-state reduction of oxides such as the it is harder to model the physicochemical process. transformation of hematite to nanocrystalline wustite.[28] McCormick has obtained main results by using single Further work includes the first reported applications of (A + BC!AB + C) or double (AB + CD!AD + BC) dis- mechanosynthesized materials to catalysis[29,30] and the placement reactions. ORDER REPRINTS 1794 Mechanosynthesis of Nanophase Powders

For instance, in the production of ultrafine Co and Ni powders of the order of 10–20 nm of many different particles[32] with a narrow particle size distribution of 10– materials. Possible research activities, related to applica- 20 nm, the reactions: tions in the field, would likely include the study of the influence of diluents in mechanochemical reactions and techniques for engineering, controlling, and sizing parti- Reaction DH (kJ/mol) cles which have dimensions 2 orders of magnitude less than the finest powders used in metal powder technologies. CoCl + 2Na ! Co + 2NaCl À495 2 More details of the technologies of nanostructured NiCl2 + 2Na ! Ni + 2NaCl À507 material synthesis by mechanical attrition, in which mechanosynthesis should be inserted, are described in were obtained by SPEX 8000 processing with a ball-to- the entry by Koch[38] in this encyclopedia; the reader is powder ratio 3:1 and with typical processing times of the referred there not only for a more thorough description order of 24 hr. Here and in other previous works, a of these technologies, but also to appreciate shortcom- remarkable effect of the diluent phase NaCl was observed ings of this methodology, mainly represented by pow- on the influence of the mean particle size, being of the der contamination during milling, which have not been order of nanometers in case 25–50 wt.% diluent and discussed here. increasing it to the micron range when not present. Further examples include the synthesis of metal sul- fide nanoparticle[33] reaction of the type ZnCl + CaS! 2 ACKNOWLEDGMENTS ZnS+CaCl2, with a mean particle size, observed by TEM, of 12 nm. The authors would like to thank Dr. Nicola Zampa for The synthesis of metal-oxide nanoparticles was de- technical help in CAD drawings; Fabio Miani would like scribed in Ref. [34]. A representative reaction is: to appreciate fruitful discussion and communication with Dr. Francesco Delogu and Prof. Carl Koch.

Reaction DH (kJ/mol)

CeCl + 3NaOH ! Ce(OH) + 3NaCl À345 3 3 REFERENCES

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